Field of the Invention
The present invention relates to a hydraulic drive device for cargo handling vehicle.
Related Background Art
A battery-powered forklift, which is one of cargo handling vehicles, is provided, for example, with a lift cylinder for moving a fork up and down, a tilt cylinder for tilting a mast, a hydraulic pump for supplying hydraulic oil to the lift cylinder and the tilt cylinder, and an electric motor for driving the hydraulic pump. The battery-powered forklift is sometimes subjected to cargo-handling regeneration as described below. During an operation of lowering a cargo with the fork, the hydraulic oil is returned from the lift cylinder to the hydraulic pump by making use of the weight of the cargo. This causes the hydraulic pump to drive the electric motor, whereby the electric motor comes to generate electric power. When the pressure of the return oil to the hydraulic pump is low, e.g., in a state in which there is no cargo on the fork (light load state), the hydraulic pump needs to be rotated by the electric motor in order to achieve a desired descent speed. In this case, the electric motor consumes power.
There is the hydraulic system described in Japanese Patent Application Laid-Open Publication No. 2-231398, which offers the technology for controlling the consumption of power. The hydraulic system described in the Publication No. 2-231398 is provided with a lift control valve arranged between a bottom chamber of the lift cylinder and the hydraulic pump, and a pilot-controlled directional control valve disposed on a branch path between the lift control valve and a tank-side passage of the hydraulic pump and configured to be switched between an open position and a close position. When the fork descends with a heavy load, the pressure of the return oil from the lift cylinder exceeds a pilot setting pressure. For this reason, the pilot-controlled directional control valve is switched to the close position, so as to forcibly feed the return oil to the hydraulic pump. When the fork descends in a state without load or with an extremely low load, the pressure of the return oil from the lift cylinder does not reach the pilot setting pressure. For this reason, the pilot-controlled directional control valve is kept at the open position and the low-pressure return oil does not flow to the hydraulic pump. The electric motor is held at a standstill.